1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor and a method of preparing the photoreceptor, and to an image forming method, an image forming apparatus and a process cartridge therefor using the photoreceptor.
2. Discussion of the Background
Recently, organic photoreceptors (OPCs) have been widely used instead of inorganic photoreceptors for copiers, facsimiles, laser printers and their complex machines because of their good performances and advantages. Specific examples of the reasons include (i) optical properties such as a wide range of light absorbing wavelength and a large amount of absorbing light; (ii) electrical properties such as high sensitivity and stable chargeability; (iii) choice of the materials; (iv) good manufacturability; (v) low cost; (vi) non-toxicity, etc.
On the other hand, as image forming apparatuses become smaller, photoreceptors have smaller diameters recently. In addition, photoreceptors are required to have high durability as image forming apparatuses produce images at a higher speed and are free from maintenance. In this respect, the organic photoreceptor typically has a soft surface layer mainly formed from a low-molecular-weight charge transport material and an inactive polymer, and therefore the organic photoreceptor typically has a drawback of being mechanically abraded with an image developer and a cleaner with ease when repeated used in the electrophotographic process. In addition, as toner particles has smaller particle diameters due to requirements for high-quality images, cleaning blades need to have higher rubber hardness and higher contact pressure for the purpose of increasing cleanability, and which also accelerates abrading photoreceptors. Such abrasions of photoreceptors deteriorate electrical properties thereof such as sensitivities and chargeabilities, and cause abnormal images such as image density deterioration and background fouling. When a photoreceptor is locally abraded, images having black stripes due to defective cleaning are produced. At present, photoreceptors are exchanged because of these abrasions and damages.
Therefore, it is indispensable to decrease the abrasion amount of the organic photoreceptor so as to have high durability. This is the most pressing issue to solve in this field.
As methods of improving the abrasion resistance of a photoreceptor, (1) Japanese Published Unexamined Patent Application No. 56-48637 discloses a photoreceptor using a hardening binder in its surface layer; (2) Japanese Published Unexamined Patent Application No. 64-1728 discloses a photoreceptor using charge transport polymer material; and (3) Japanese Published Unexamined Patent Application No. 4-281461 discloses a photoreceptor having a surface layer wherein an inorganic filler is dispersed. The photoreceptor using a hardening binder of (1) tends to increase a residual potential and decrease image density because of a poor solubility of the binder with a charge transport material and impurities such as a polymerization initiator and an unreacted residual group. The photoreceptor using charge transport polymer material of (2) and the photoreceptor having a surface layer wherein an inorganic filler is dispersed of (3) have abrasion resistance to some extent, but which is not fully satisfactory. Further, the photoreceptor having a surface layer wherein an inorganic filler is dispersed of (3) tends to increase a residual potential and decrease image density because of a trap present on the surface of the inorganic filler. Any of the photoreceptors of (1) to (3) does not have fully satisfactory integrated durability such as electrical durability and mechanical durability.
To improve the abrasion resistance of the photoreceptor of (1), Japanese Patent No. 3262488 discloses a photoreceptor including hardened urethane acrylate. However, although disclosing that the photosensitive layer includes the hardened urethane acrylate, Japanese Patent No. 3262488 only discloses that a charge transport material may be included therein and does not disclose specific examples thereof. When a low-molecular-weight charge transport material is simply included in a photosensitive layer, the low-molecular-weight charge transport material is not soluble with the hardened urethane acrylate and the low-molecular-weight charge transport material separates out, and which causes deterioration of mechanical strength of the resultant photoreceptor such as a crack. In addition, Japanese Patent No. 3262488 discloses that a polycarbonate resin is included in the photosensitive layer to improve the solubility. However, a content of the hardened urethane acrylate decreases, resulting in insufficient abrasion resistance of the photoreceptor. A photoreceptor not including a charge transport material in its surface layer, which is thin against deterioration of potential of the irradiated part, has a short life. In addition, the charged potential thereof has poor stability against environment.
As an abrasion resistance technology of a photosensitive layer in place of these technologies, Japanese Patent No. 3194392 discloses a method of forming a charge transport layer using a coating liquid formed from a monomer having a carbon-carbon double bond, a charge transport material having a carbon-carbon double bond and a binder resin. The binder resin includes a binder resin having a carbon-carbon double bond and a reactivity with the charge transport material, and a binder resin having neither a carbon-carbon double bond nor a reactivity with the charge transport material. The photoreceptor has good abrasion resistance and electrical properties. However, when a binder resin not having a reactivity with a charge transport material, such as an acrylic polymer, a styrene polymer, an acrylic styrene copolymer, a polyester resin, a polycarbonate resin and an epoxy resin, a bonding amount between the monomer having a carbon-carbon double bond and the charge transport material having a carbon-carbon double bond decreases, resulting in insufficient crosslink density of the photosensitive layer. Further, since the binder resin itself does not have toughness, the resultant photosensitive layer does not have satisfactory abrasion resistance.
Japanese Published Unexamined Patent Application No. 2000-66425 discloses a photosensitive layer including a hardened positive hole transport compound having two or more chain polymerizable functional groups in the same molecule. However, since the photosensitive layer includes a bulky positive hole transport material having two or more chain polymerizable functional groups, a distortion appears in the hardened compound and an internal stress increases to cause a roughness and a crack of the surface layer, resulting in insufficient durability of the resultant photoreceptor.
Japanese Published Unexamined Patent Applications Nos. 2004-302450, 2004-302451 and 2004-302452 disclose a crosslinked charge transport layer in which a tri- or more functional radical polymerizing monomer having no charge transport structure and a monofunctional radical polymerizing compound having a charge transport structure are hardened, wherein the monofunctional radical polymerizing compound having a charge transport structure improves mechanical and electrical durability of the layer and prevents the layer from being cracked. Particularly, the photoreceptor disclosed in Japanese Published Unexamined Patent Application No. 2004-302452 has smaller surface roughness with a specified amount of a multifunctional acrylic monomer for the purpose of having good cleanability and preventing production of abnormal images. However, since the multifunctional acrylic monomer has a large volume contraction, the surface layer insufficiently adheres to the lower photosensitive layer. When such a photoreceptor is used in an image forming apparatus wherein a large mechanical stress is applied thereto, the crosslinked surface layer separates from the photosensitive layer, resulting in inability of maintaining sufficient abrasion resistance for long periods.
Because of these reasons, a need exists for an electrophotographic photoreceptor having high durability, good and stable electrical properties and good cleanability for long periods.